Laser scanner with amplitude and phase detection

ABSTRACT

A method for optical evaluation of a sample includes scanning a beam of coherent radiation over the sample, whereby the radiation is scattered from the sample, while directing a portion of the scanning beam toward a diffraction grating so that the portion of the beam is scanned over the grating, whereby a frequency-shifted reference beam is diffracted from the grating. The scattered radiation and the frequency-shifted reference beam are combined at a detector to generate an optical heterodyne signal.

RELATED APPLICATION

The present application is a Divisional of U.S. patent application Ser.No. 10/232,093, filed Aug. 29, 2002 now U.S. Pat. No. 6,937,343,entitled, “Laser Scanner With Amplitude and Phase Detection”. Thispatent application is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to laser scanning systems, andspecifically to methods and systems for optical inspection of substratesbased on laser scanning.

BACKGROUND OF THE INVENTION

Heterodyne detection is a well-known method of optical signalprocessing. It is described, for example, by Vanderlugt in OpticalSignal Processing (John Wiley & Sons, New York, 1992), Chapters 9-10,which are incorporated herein by reference. Typically, to performheterodyne detection, a laser beam is split into a probe beam and areference beam. The reference beam, of amplitude A₀, isfrequency-shifted by a known carrier frequency f_(c), typically using anacousto-optic modulator operating in the radio frequency (RF) range. Theprobe beam is incident on the sample, and is modified in amplitude andphase as a result, so that the beam reflected (or transmitted) by thesample has amplitude A₁(t) and phase φ(t). The probe and reference beamsare recombined and mutually interfere to give an optical signal whoseintensity has the form:I(t)=A ₀ ² +A ₁ ²+2A ₀ A ₁(t) cos [2πf _(c) t−φ(t)]  (1)

This combined signal is incident on a detector, and the detector outputis filtered to extract the signal component at frequency f_(c). Thisheterodyne component is linear in the amplitude change A₁(t) caused bythe sample, and also contains the phase change data Φ(t). Therefore,information regarding the structure and characteristics of the sample istypically more easily extracted from the heterodyne signal than fromsimple (homodyne) intensity-based detection.

Interferometric measurements are known in the art of optical inspectionof patterned substrates, such as semiconductor wafers. For example, U.S.Pat. No. 6,052,478, whose disclosure is incorporated herein byreference, describes an automated photomask inspection apparatus thatuses transmitted or reflected interferometry to measure phase shiftsproduced by such masks. Variations in the phase shifts are indicative ofdefects due to undesired thickness variations in the photomask.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to providelaser scanning systems that can be used to provide information regardingboth the reflectivity of a sample and phase variations induced by thesample surface.

It is a further object of some aspects of the present invention toprovide an improved laser scanning system for use in heterodynedetection.

In some preferred embodiments of the present invention, a scanning laserbeam is split into probe and reference beams. The probe beam is focusedonto the surface of a sample, such as a semiconductor wafer, while thereference beam is focused to a small spot on a grating. As the laserbeam is scanned across the sample, it simultaneously scans across thesurface of the grating, causing the reference beam to be diffracted fromthe grating with a phase shift that varies over time as the beam isscanned. This time-varying phase shift is equivalent to modulating thereference beam at a frequency that is proportional to the scanning speedand grating pitch. The probe beam scattered from the sample is combinedwith the frequency-modulated reference beam at a detector. The amplitudeof the resultant heterodyne signal is indicative of the reflectivity ofthe sample, while variations in the phase of the heterodyne signalrepresent phase changes caused by the sample in the scattered probebeam.

Thus, preferred embodiments of the present invention allow heterodynedetection to be implemented simply in a laser scanning system, withoutrequiring an active modulator to modulate the reference beam. Instead,the same scanning device that is used to scan the probe beam across thesample also provides the means for modulating the reference beam, usingonly a passive, stationary grating and associated optics.

In other preferred embodiments of the present invention, a homodynedetection scheme is used to measure the reflectivity and phasecharacteristics of the sample. In this case, the reference beam isreflected from a planar mirror before being combined with the probe beamscattered from the sample. The combined beam is split into phase andquadrature components, preferably by means of suitable beam retardationand polarization optics. The time-varying amplitudes of the phase andquadrature beam components are measured by respective detectors, whilethe probe beam scans over the sample. The phase and quadrature signalsoutput by the detectors are then processed together in order to separatethe amplitude (reflectivity-related) and phase information carried bythe scattered beam.

There is therefore provided, in accordance with a preferred embodimentof the present invention, a method for optical evaluation of a sample,including:

scanning a beam of coherent radiation over the sample, whereby theradiation is scattered from the sample;

directing a portion of the scanning beam toward a diffraction grating sothat the portion of the beam is scanned over the grating, causing afrequency-shifted reference beam to be diffracted from the grating; and

combining the scattered radiation and the frequency-shifted referencebeam at a detector to generate an optical heterodyne signal.

Preferably, scanning the beam includes scanning the beam laterally witha predetermined scanning speed, causing the reference beam to be shiftedby a carrier frequency that is proportional to the scanning speed.Further preferably, the grating has a predetermined pitch, causing thereference beam to be shifted by a carrier frequency that is proportionalto the pitch of the grating.

In a preferred embodiment, the diffraction grating includes a Littrowgrating, and directing the portion of the scanning beam toward thediffraction grating includes directing the portion of the scanning beamtoward the Littrow grating along a predetermined beam direction, so thatthe grating returns the frequency-shifted reference beam substantiallyparallel to the predetermined beam direction. In a further preferredembodiment, directing the portion of the scanning beam toward thediffraction grating includes dividing the portion of the scanning beaminto multiple diffraction orders, including a zero order, and directingone of the diffraction orders other than the zero order toward thediffraction grating.

Preferably, the method includes detecting and processing the opticalheterodyne signal responsive to a known carrier frequency of thereference beam, so as to derive amplitude and phase information from thescattered radiation. Most preferably, processing the optical heterodynesignal includes processing the amplitude and phase information todetermine a property of a surface of the sample from which the radiationis scattered. In a preferred embodiment, the sample includes asemiconductor wafer, and processing the amplitude and phase informationincludes processing the information to detect a defect on the surface ofthe wafer.

There is also provided, in accordance with a preferred embodiment of thepresent invention, apparatus for optical evaluation of a sample,including:

a radiation detector, adapted to detect an optical heterodyne signal;

a scanner, adapted to scan a beam of coherent radiation over the sample,whereby the radiation is scattered from the sample;

a diffraction grating;

a beamsplitter, aligned with the scanned beam so as to direct a portionof the beam toward the diffraction grating so that the portion of thebeam is scanned over the grating, causing a frequency-shifted referencebeam to be diffracted from the grating; and

collection optics, positioned to combine the scattered radiation and thefrequency-shifted reference beam to generate the optical heterodynesignal at the detector.

Preferably, the apparatus includes a signal processor, coupled to detectand process the optical heterodyne signal responsive to a known carrierfrequency of the reference beam, so as to derive amplitude and phaseinformation from the scattered radiation.

There is additionally provided, in accordance with a preferredembodiment of the present invention, a method for optical evaluation ofa sample, including:

scanning a beam of coherent radiation over the sample, whereby theradiation is scattered from the sample;

splitting off a portion of the beam to serve as a reference beam;

combining the scattered radiation and the reference beam so as togenerate a combined beam that is characterized by interference betweenthe scattered radiation and the reference beam;

separating the combined beam into first and second beam componentshaving a predetermined phase difference therebetween; and

comparing respective time variations of the first and second beamcomponents so as to derive amplitude and phase information from thescattered radiation.

Preferably, separating the combined beam includes splitting the combinedbeam into phase and quadrature components.

There is further provided, in accordance with a preferred embodiment ofthe present invention, apparatus for optical evaluation of a sample,including:

a scanner, adapted to scan a beam of coherent radiation over the sample,whereby the radiation is scattered from the sample;

a first beamsplitter, aligned with the scanned beam so as to separateoff a portion of the beam to form a reference beam, which is notscattered from the sample;

collection optics, positioned to combine the scattered radiation and thereference beam so as to generate a combined beam that is characterizedby interference between the scattered radiation and the reference beam;

a second beamsplitter, operative to separate the combined beam intofirst and second beam components having a predetermined phase differencetherebetween;

first and second detectors, positioned to receive the first and secondcomponents, respectively, and adapted to generate first and secondsignals responsive thereto; and

a signal processor, which is coupled to receive the first and secondsignals and to compare respective time variations of the signals so asto derive amplitude and phase information regarding the scatteredradiation.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a laser scanner system withheterodyne detection, in accordance with a preferred embodiment of thepresent invention;

FIG. 2 is a schematic detail view of a grating used in the system ofFIG. 1; and

FIG. 3 is a schematic side view of a laser scanner system with homodynedetection, in accordance with a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is a schematic side view of a system 20 for laser scanning of asubstrate 22, in accordance with a preferred embodiment of the presentinvention. Typically, substrate 22 comprises a semiconductor wafer,which is scanned by system 20 in order to detect defects on the wafersurface. Alternatively, the principles embodied in the system may beapplied to other fields of optical inspection. A laser 24 generates abeam of coherent light, which is expanded by a beam expander 25 and israpidly deflected by a scanner 26, such as an acousto-optic scanner,galvanometer-based scanner or rotating polygonal mirror, as is known inthe art. Scanner 26 preferably scans the laser beam at high angularspeed, typically covering a range of about 100 mrad (milliradians) in ascan time on the order of 25 μs. Two beams 27 and 29 are shown reflectedfrom scanner 26, representing approximately the beam positions at theextremes of the scan.

The scanned laser beam is focused by an input telescope 30 and is thensplit by a beamsplitter 28 into probe and reference beams. The probebeam is focused by a probe objective assembly 32 onto the surface ofsubstrate 22. The objective assembly preferably comprises an objectivelens 31 with a telecentering stop 33, used to maintain uniformity of thefocal spot on substrate 22 over the entire scan line. Telescope 30 ispreferably configured to image the pivot point of the beam at scanner 26onto stop 33.

Light scattered from the surface is collected by objective assembly 32and is directed back toward beamsplitter 28. As the beam scans over thesurface of the substrate, the amplitude of the scattered light ismodulated by the varying reflectivity of the substrate. The phase of thescattered light also varies, due to microscopic variations in thecomposition and elevation of the surface. These amplitude and phasevariations may occur due to features intentionally formed on thesurface, such as patterns that are deposited on semiconductor wafers inthe course of integrated circuit manufacture, or due to defects.

The reference beam is focused by a reference objective 34 onto a firstdiffraction grating 37. This first grating is positioned and blazed sothat a zero order 39 of the grating is discarded, while a first order 41is incident on a second diffraction grating 36. The second grating ispreferably a Littrow grating operating in first order. Second grating 36is blazed and aligned so that the first-order light that it diffracts isdirected back toward first grating 37, which then diffracts this lighttoward beamsplitter 28 parallel to the incident reference beam. Scanner26 causes the reference beam to scan across the surface of the grating(vertically in the view shown in the figures), with the result that thediffracted beam is frequency-shifted at a carrier frequency f_(c). Thefrequency shifting mechanism is described in detail below with referenceto FIG. 2.

Preferably, beamsplitter 28 is a polarizing beamsplitter, causing thereference beam (reflected toward the right) to be P-polarized, while theprobe beam (passing through the beamsplitter) is S-polarized. Typically,the beam emitted by laser 24 is linearly polarized, and the beampolarization is oriented relative to beamsplitter 28 so that most of thebeam power passes to the probe beam. (Alternatively, a non-polarizingbeamsplitter may be used, though at the expense of lower efficiency.)The polarized probe and reference beams pass through respectivequarter-wave plates 35 on both their forward and return paths. Inconsequence, the returned probe beam becomes P-polarized, so that itpasses directly through beamsplitter 28, while the returned referencebeam becomes S-polarized, so that it is reflected to the left by thebeamsplitter.

An output telescope 38 focuses both the scattered probe beam and thediffracted reference beam onto a detector 40, causing the beams tointerfere at the detector. If the probe and reference beams areorthogonally polarized, as described above, a polarizer 43, oriented at45° between the S and P polarization directions, is interposed in frontof detector 40 in order to engender the desired interference. Telescope38 is preferably configured to image stop 33 onto detector 40. Onaccount of the modulation of the reference beam, the signal received bydetector 40 has a heterodyne component at the carrier frequency f_(c),as given above by equation (1). A signal processor 42, typically ageneral-purpose computer with suitable front-end electronics, filtersand analyzes the heterodyne signal component, in order to measure theamplitude and phase variations created in the probe beam due to featuresand defects on the surface of the sample.

FIG. 2 is a schematic detail view of grating 36, showing how an incidentreference beam 50 is frequency-modulated by scanning across the grating.As shown in the figure, grating 36 is preferably a Littrow grating,which is blazed so as to reflect a first-order beam 54 at the specifiedlaser wavelength, parallel to the incident beam. Alternatively, thegrating may be configured so that a second- or higher-order beam isdiffracted back parallel to the incident beam. Other grating types andgeometries may be used, as well. For example, system 20 may beconfigured in the form of a Mach-Zehnder interferometer, as is known inthe art, in which case grating 36 may be replaced by a transmissivegrating, or another type of reflective diffraction grating may be used.In such cases, the diffracted beam is not necessarily parallel to theincident beam over its entire path as in the present embodiment.

In the embodiment shown in FIG. 2, the front surface of grating 36comprises parallel teeth 56 with a grating period d and a blaze heighth. To provide the required constructive interference in first-order beam54, the blaze height is equal to one-half wave at the laser wavelength.Thus, as the incident reference beam scans over the grating surface by adistance d, the phase of the first-order beam varies cyclically through2π. Assuming the incident beam scans over the surface at a linearvelocity v, the modulation frequency of the first-order beam is simplyf_(c)=v/d, i.e., the number of teeth 56 traversed in one second.

For effective extraction of information regarding substrate 22 from theheterodyne signal by processor 42, it is desirable that f_(c) besubstantially greater than the information bandwidth. Assuming referenceobjective 34 to have focal length L, the modulation frequency as afunction of the parameters of scanner 26 and grating 36 is given by:

$\begin{matrix}{f_{c} = {\frac{scan\_ angle}{scan\_ time} \cdot \frac{L}{d}}} & (2)\end{matrix}$

For L=50 mm, with a scan angle of 100 mrad, scan time 25 μs, and d=2 μm(grating pitch 500 line pairs/mm), f_(c)=100 MHz. This frequency issufficient for use in high-speed wafer inspection systems, for example,which typically scan wafers at rates on the order of tens of millions ofspots per second.

FIG. 3 is a schematic side view of a system 58 for laser scanning ofsubstrate 22, in accordance with another preferred embodiment of thepresent invention. System 58 is similar in certain aspects to system 20,as shown and described above, except that system 58 operates byhomodyne, rather than heterodyne, detection. The reference beam splitoff by beamsplitter 28 is focused by objective 34 onto a planar mirror60, which thus reflects the reference beam back upon itself withconstant phase delay. An output lens 62 collects both the scatteredprobe beam and the diffracted reference beam into a detectionbeamsplitter 64. Focusing lenses 66 direct the probe and reference beamsfrom beamsplitter 64 together onto detectors 68 and 70, so that thebeams interfere at the detectors.

As described above with reference to FIG. 1, the probe and referencebeams are preferably orthogonally polarized. Therefore, polarizers 72,oriented at 45° relative to the S and P polarization directions, areinterposed in the paths of the beams following beamsplitter 64, so as toengender interference between the beams. Alternatively, if beamsplitter64 is a polarizing beamsplitter, with its polarization axis oriented at45° relative to the S and P directions of the probe and reference beams,polarizers 72 may be eliminated.

A quarter-wave plate 74 is inserted in the beam path to detector 68.Plate 74 has the effect of shifting the relative phases of the S and Pbeams by 90°, so that the interfering beams at one of detectors 68 and70 are “in phase,” while the interfering beams at the other detector arein “quadrature.” (For convenience, it will be assumed that the in-phasecomponent is incident on detector 68, while the quadrature component isincident on detector 70, but these designations are arbitrary.) Thein-phase signal output by detector 68 can be expressed as:I(t)=A ₀ ² +A ₁ ²2A ₀ A ₁(t) cos [φ(t)]  (3)

-   -   while the quadrature signal output by detector 70 is:        Q(t)=A ² ₀ +A ₁ ²(t)+2A ₀ A ₁(t) sin [φ(t)]  (4)

By comparing these two signals, processor A2 is able to separate theamplitude component A₁(t) and the phase component φ(t) of theinterfering beams. These components are indicative of the amplitude andphase variations created in the probe beam due to features and defectson the surface of the sample.

Although the preferred embodiments described above are directed tobright field detection of light reflected from a sample, the principlesof the present invention may be applied, mutatis mutandis, to dark fielddetection schemes, as well as to transmission-based measurements. Indark field inspection with heterodyne detection, for example, atwo-dimensional, non-Littrow grating can be used to generate thefrequency-shifted reference beam at desired angles. These variousdetection schemes are useful not only in observing defects and patternvariations in semiconductor wafers and photomasks, but also in a widerange of other applications of optical heterodyne and homodynedetection, such as in scanning microscopy, including particularlyconfocal microscopy.

It will thus be appreciated that the preferred embodiments describedabove are cited by way of example, and that the present invention is notlimited to what has been particularly shown and described hereinabove.Rather, the scope of the present invention includes both combinationsand subcombinations of the various features described hereinabove, aswell as variations and modifications thereof which would occur topersons skilled in the art upon reading the foregoing description andwhich are not disclosed in the prior art.

1. A method comprising: projecting a scanning beam of coherent radiationtoward a surface of a sample to be optically evaluated; splitting offand polarizing a first portion of the scanning beam to serve as apolarized scanning reference beam; polarizing a remaining portion of thescanning beam and focusing a resulting polarized scanning probe beamonto the sample so as to produce scattered radiation from the sample;combining the scattered radiation and the reference beam so as togenerate a combined beam that is characterized by interference betweenthe scattered radiation and the reference beam; separating the combinedbeam into an in phase component and a quadrature component having a 90degree phase difference therebetween, the in phase and quadraturecomponents each characterized by interference between the scatteredradiation and the reference beam; detecting the in phase component andthe quadrature component of the combined beam and generating respectivein-phase and quadrature signals responsive thereto; and detectingfeatures and defects on the surface of the sample by comparing the inphase and quadrature signals so as to derive amplitude and phaseinformation indicative of amplitude and phase variations created in thescattered radiation.